1. Field of the Invention
The present invention is directed to a solid-state image sensor, and relates particularly to a solid-state image sensor equipped with means for injecting bias charges.
A two-layered solid-state image sensor having a photoconductive film deposited on a solid-state image sensing element or a stacked solid-state image sensor can have a wide opening area of its photosensitive part, it has good characteristics of a high sensitivity and low smear. Because of this feature, this type of solid-state image sensor is considered promising as a camera for various monitor televisions or a high definition television. An amorphous film, such as an Se-As-Te film, ZnSe-ZnCdTe film or a-Si:H (amorphous silicon hydride) film, is used at present as a photoconductive film of such a stacked solid-state image sensor. Of these materials, an a-Si:H film is becoming popular due to its good characteristic and workability as well as its property to be formed at a low temperature.
With the use of a conventional solid-state image sensor as disclosed in, for example, IEEE Transaction on Electron Device, Vol. ED-32, No. 8, August 1985, light incident from a transparent electrode is photoelectrically converted by a photoconductive film to thereby provide electron-hole pairs. Since the potential of a pixel electrode electrically coupled to a storage diode is higher than that of the transparent electrode, electrons move toward the pixel electrode and holes move toward the transparent electrode. The holes flow out to an external circuit through the transparent electrode whereas the electrons are accumulated in the storage diode coupled to the pixel electrode t thereby reduce the potential of this diode. Signal charges (electrons) accumulated for a given period of time are read out on a vertical CCD channel from the storage diode when a signal charge read-out pulse is applied to a signal charge read-out gate. The charges transferred onto the vertical CCD channel are output through a horizontal CCD channel.
The above conventional solid-state image sensor has the following shortcomings. Since the storage diode is electrically coupled to a pixel electrode wire filled with free electrons, the storage diode would not be completely depleted and the transfer of the signal charges would be incomplete. This is likely to produce a capacitive lag. In addition, since the charges photoelectrically converted w thin the photoconductive film are trapped by the trapping level existing in the film and are discharged after a certain period of time, the residual image characteristic of the solid-state image sensor is impaired.
The residual image can be reduced by injecting bias charges in the storage diode; for a television camera employing an image pick-up tube, for example, the injection of the bias charges is done using light from a light source. With this method, however, it is significantly difficult to uniformly irradiate light on the chip so that a residual image cannot surely be reduced. Furthermore, the need for a separate light source complicates the structure of the solid-state image sensor.
As described above, according to the conventional solid-state image sensor transferring the signal charges from the storage diode is incomplete and a capacitive lag is produced. The conventional method for reducing a residual image by injecting bias charges by means of a light source does not ensure uniform injection of bias charges and complicates the structure of the image sensor.